Transcript Ch12Pres - Leornian.org

AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 12
Tropical Weather Systems
© AMS
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Case-In-Point
 Hurricane Katrina – 28 August 2005
– Devastated the Gulf Coast of Louisiana and Mississippi; New Orleans
experienced catastrophic flooding
– 3rd most intense landfalling U.S. hurricane
– Claimed 1,300 lives and is the most destructive hurricane in terms of
economic loss
– New Orleans topography made the city particularly vulnerable
 The city occupies a bowl between the Mississippi River and Lake
Pontchartrain, much of which is up to 1.8 m (6 ft) below sea level
 New Orleans relies on levees and pumps to keep water out
 Levee system was breached, pumps failed, and city was flooded to
depths of up to 6 m (20 ft)
– New Orleans residents were warned in advance, but thousands did
not evacuate
– Katrina was followed by Hurricane Rita less than a month later
 Landfall was well to the west of New Orleans (near Sabine Pass, TX), but
flood waters up to 1.5 m (5 ft) deep spread over parts of the city due to
new breaks in the levees
– Hurricane Gustav hit the Gulf Coast in 2008, causing $4.3 billion in
damage
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Driving Question
 What conditions are required for the
development of tropical cyclones?
– This chapter will describe:
 Weather in the tropics
 Characteristics of tropical cyclones
 Geographical and seasonal distribution
 Associated hazards
 Tropical cyclone life cycle
 Forecasting efforts
 Unsuccessful experiments to modify these storms
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Weather in the Tropics
 The tropics is the belt between the Tropic of Cancer (23.5 degrees N)
and the Tropic of Capricorn (23.5 degrees S)
 Weather exhibits very little seasonal variation with uniformly high
temperatures
 Diurnal temperature variation is typically greater than the range of
monthly mean temperatures over the course of a year
 No fronts or frontal weather
– Air masses are uniformly warm and humid
 Thunderstorm activity
– Thunderstorms may align in tropical non-squall clusters
– More intense cells (tropical squall clusters) can form that look like middle
latitude squall lines
– ITCZ stimulates thunderstorm activity and follows the sun, so that summer is
the rainy season and winter is the dry season
 Very little horizontal pressure gradient
– Isobaric analysis is of little value
– Streamline analysis is used instead
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 A streamline is a line drawn on a map that is parallel to the wind direction
 Can be used to identify regions of divergence and convergence, such as that
associated with an easterly wave
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Hurricane Characteristics
 Definition – an intense cyclone that originates over
tropical ocean waters, usually in late summer to
early fall and has a maximum sustained wind speed
≥119 km per hr (74 mph)
 Contrast with an Extra-tropical Cyclone
– No associated fronts or frontal weather due to its origin
over uniformly warm and humid conditions
– Sea level pressure and steep horizontal air pressure
gradient are typically greater than that of an extra-tropical
cyclone
– A hurricane is a much smaller system
– A mature hurricane is a warm-core low whose circulation
weakens with altitude; an extra-tropical cyclone is a coldcore low whose circulation strengthens with altitude
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Hurricane Characteristics
 The eye is at the center of a hurricane
– Almost cloudless skies, subsiding air, light winds
– Diameter ranging from 10 to 65 km (6 to 40 mi)
– Eye shrinks as hurricane intensifies
 The eye wall (borders the eye of a mature storm)
– Ring of cumulonimbus clouds that produce heavy rains and very
strong winds
– The most dangerous and potentially most destructive part of a
hurricane is the eye wall on the side of the advancing system where
the wind blows in the same direction as the storm’s forward motion
 In the Northern Hemisphere, this occurs on the right side of the
hurricane when facing in the direction of the system’s forward motion
 Cloud bands spiral inward towards the eye wall and produce
heavy convective showers and hurricane-force winds
 At high altitudes, cirrus or cirrostratus spiral outward
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Hurricane Characteristics
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Hurricane Characteristics
The eye wall of Hurricane Katrina on 28
August 2005 while the storm was over the
Gulf of Mexico
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Where and When
 Necessary conditions for formation:
– Relatively high sea surface temperatures (SST)
 SST of at least 26.5 °C (80 °F) through an ocean depth of 45 m (150 ft)
or more
 Sustains circulation by the latent heat released when water vapor,
evaporated from the ocean surface, is conveyed upward and condenses
 Strong tropical cyclone winds can induce Ekman transport and lead to
lower SST
 Cyclones may intensify over warm-core rings and weaken over cold-core
rings
 SST requirement makes formation seasonal
– Most Atlantic hurricanes develop in late summer and early autumn
when ocean surface waters are warmest
– Official season runs from 1 June to 30 November
– Peak threat to U.S. coastline is from mid-August to late October
– Adequate Coriolis Effect
 With rare exception, tropical cyclones do not form within 5 degrees of
the equator
– Weak vertical wind shear
 Allows cluster of cumulonimbus to form
 Wind shear would tear these apart
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– Relatively humid air in the mid-troposphere
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Tropical Cyclone Breeding Grounds
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Where and When
Frequency of Atlantic basin tropical storm and hurricanes
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Where and When
 During El Niño, Atlantic hurricanes are infrequent due to
strong high altitude winds
 Strong wind shear is the main reason why hurricanes
rarely form off the coasts of South America
 Worldwide average of 80 tropical cyclones per year
– ½ strengthen to hurricanes
– The western Pacific Ocean is the most active area
 About 27 systems each season, about 17 of which intensity into
typhoons (~ 4 supertyphoons)
 Only hurricanes spawned in the tropical Atlantic, Caribbean
and Gulf of Mexico pose a serious threat to coastal North
America
– Average – 10.6 named tropical storms, 6 become hurricanes (2.4
major hurricanes), 2.5 hurricanes strike U.S. coast each year
– 2005 season set a record with 27 named tropical storms
– Every Atlantic/Gulf coast state from TX to ME has been hit
 FL is most hurricane prone, TX 2nd, LA 3rd
– The Pacific coast is rarely a hurricane target due to NE trades
© AMS – The Hawaiian Islands are sometimes threatened
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Life Cycle of a Hurricane
 Tropical Disturbance
– An organized cluster of
cumulonimbus clouds over
tropical seas that has a
surface center of low
pressure; usually triggered
by the ITCZ
– Easterly Wave
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 A ripple in the tropical
easterlies featuring a weak
trough of low pressure
 Forms over East Africa and
propagates westward
 Precursors of ~ 65% of
named Atlantic tropical
cyclones
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Life Cycle of a Hurricane
 Tropical Disturbance
– Only a small percentage of convective cloud clusters in the tropical
Atlantic evolve into full-blown hurricanes
 Subsidence of air on the eastern flank of the Bermuda-Azores
anticyclone and trade-wind inversion inhibit deep convection
 Vertical wind shear is usually too great
 Atmospheric conditions that inhibit cyclone formation appear to be
associated with the Sahara air layer (SAL), an elevated mass of dry,
dusty, stable air originating over the Sahara Desert that travels many
thousands of kilometers westward over the Atlantic
– If conditions favor hurricane development, the surface air pressure
falls and a cyclonic circulation develops
 Water vapor condenses within the storm, releases latent heat, and the
heated air rises. Expansional cooling of rising air triggers more
condensation and release of latent heat
 Rising temperatures in the storm’s core and divergence of air aloft
trigger a sharp drop in surface air pressure and increased surface
convergence
 If favorable conditions persist, cycle continues and the tropical
disturbance intensifies and its winds strengthen
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Life Cycle of a Hurricane
 Tropical Depression
– Maximum sustained winds reach 37 km per hr
(23 mph) or higher
 Tropical Storm
– Winds reach at least 63 km per hr (39 mph)
– Assigned a name
 Hurricane
– Winds reach 119 km per hr (74 mph) or higher
 As storm weakens, it is downgraded by
reversing this classification system
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Life Cycle of a Hurricane
 Tropical cyclone trajectories are
often erratic, however, cyclones
typically initially drift westward
and curve toward the north and
northeast when they reach the
western Atlantic
 Upon reaching about 30
degrees N, a hurricane may
begin to acquire extra-tropical
characteristics as colder air is
drawn into the system and fronts
develop
 Some hurricanes fueled by the
warm Gulf Stream may maintain
tropical characteristics far up the
Atlantic coast
 New England has been the
target of many strong hurricanes
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Hurricane Hazards
 Heavy rains and inland flooding
– Freshwater flooding was responsible for 60% of deaths
from 1970 – 1999 attributed to tropical cyclones or their
remnants
 Strong winds
– Responsible for 12% of deaths during the same period
 Tornadoes
 Storm surge
– Caused most of the 1,300 fatalities associated with
Hurricane Katrina
– Remains the most serious potential impact of a
landfalling hurricane
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Hurricane Hazards
 Inland Flooding
– Rains are typically 13 - 25
cm (5 - 10 in.)
– Heavy rains persist as the
storm tracks inland
– Case study – Hurricane
Agnes (1972)
 Rains accounted for most of
the property damage
 Devastating floods in midAtlantic region, especially
central Pennsylvania, when
heavy rain fell on already
saturated grounds and hilly
terrain
– Hurricane Mitch (1998),
Tropical Storm Alberto
(1994), and Tropical Storm
Allison (2001) also caused
devastating flooding
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Hurricane Hazards
 Inland Flooding
– The image shows
radar-determined
cumulative rainfall
over southeast
Texas produced by
the remnants of
Tropical Storm
Allison
– Allison ranks as the
most deadly and
costly tropical storm
to strike the U.S.
mainland
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Hurricane Hazards
 Wind
– Wind pressure, the force per unit area caused by air in
motion, increases with the square of the wind speed
– Debris transported by wind increases damage potential
– Small but powerful whirlwinds (spin-up vortices)
embedded in a hurricane’s circulation may be
responsible for the most severe property damage
– Winds diminish rapidly upon storm landfall
 Hurricane over land is no longer in contact with warm ocean
water, its energy source
 Frictional resistance slows wind and shifts wind direction toward
the center; causes the storm to fill and weaken
 The system may still produce tornadoes after making landfall,
partially due to strong wind shear between the surface and aloft
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Hurricane Hazards
 Storm surge
– A dome of ocean water 80 – 160 km (50 – 100 mi) wide that sweeps over
the coastline near the hurricane’s landfall
– Caused by strong winds and low barometric pressure and is most likely on
the side of the hurricane with onshore winds
– Wind-driven waves on top of the dome of water, armed with floating debris,
are responsible for much of the structural damage
– Prior to 1970, was responsible for the majority of hurricane-related
fatalities. Awareness, warnings, and evacuation have generally been much
better since then.
– From 1970 to 1999, there were only 6 storm surge deaths. Then there was
Hurricane Katrina and the death and destruction caused by its surge.
– 1895 unnamed hurricane killed an estimated 2,000, and left 20,000 to
30,000 homeless due to the storm surge
– The most deadly U.S. natural disaster was the hurricane that hit Galveston,
TX (1900) when 8,000 people perished
– Hurricane Camille (1969) produced a 7.3 m (24.3ft) surge at Pass
Christian, MS
– East Pakistan (now Bangladesh) in 1970 – storm surge killed ~300,000
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Hurricane Hazards
 What causes a storm surge?
– Strong onshore winds combined with low air pressure
 Low air pressure causes the water to rise about 0.5 m for every
50 mb drop in pressure (or about 1 ft for every 1 in. of mercury
drop in pressure)
– The storm surge is superimposed on top of normal tides
– A surge of 1 – 2 m (3 – 6.5 ft) can be expected with a
weak hurricane; that of a violent hurricane may top 5 m
(16.4 ft)
– The greatest potential for a surge occurs with strong
onshore winds, a shallow sloping shoreline, during high
tide, and in densely-populated areas lacking coastal
buffers
– Storm surges are accurately predicted with a numerical
model called SLOSH (Sea, Lake, and Overland Surges
from Hurricanes)
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Storm Surge
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Saffir-Simpson Hurricane Intensity Scale
 Provides an estimate of potential coastal flooding and property damage
from a hurricane landfall
 Wind speed is the primary determining factor for a hurricane’s rating
 Storm surge is just an estimate
– It depends on underwater topography and other factors in the region of
landfall
 Property damage rises rapidly with rating
– 100-300 times greater damage from a category 4 or 5 hurricane than a
category 1
 From 1901 to 2004, about 37% of landfalling hurricanes were classified
as major (category 3 or above)
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Saffir-Simpson Hurricane Intensity Scale
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Saffir-Simpson Hurricane Intensity Scale
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Saffir-Simpson Hurricane Intensity Scale
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Trends in Hurricane Frequency
 The figure shows the
recent upward trend in the
number of intense
(category 4 and 5)
hurricanes worldwide,
even though the overall
number of hurricanes
worldwide has declined
since the 1990s
 Most of the increased
frequency occurred in the
North Pacific, SW Pacific,
and Indian Ocean, with
only a small increase in
the North Atlantic
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Trends in Hurricane Frequency
 The North Atlantic is the
only region of the globe
where overall tropical
cyclone activity has
increased recently
 Several factors contribute:
– Higher SST in the tropical
Atlantic related to the
Atlantic Multidecadal
Oscillation (AMO)
– An amplified ridge over the
central and eastern North
Atlantic
– Weaker vertical wind shear
in the deep tropics over the
central North Atlantic
– Favorable African easterly
jet stream
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Hurricane Threat to the Southeast
 The infrequency in major
hurricanes during the 1970s and
1980s lulled many coastal
residents of the southeast U.S.
into a false sense of security
and encouraged coastal
development and growth
 In 2005, NOAA reported that the
coast is home to 53% of all
Americans
 Population growth is most rapid
from Texas through the
Carolinas, especially in Florida
 Public safety officials are
concerned about the trend
toward more Atlantic tropical
cyclones
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Hurricane Threat to the Southeast
Tracks of three of the four hurricanes that struck Florida in 2004
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Hurricane Threat to the Southeast
 Barrier islands are particularly at risk
– A barrier island is an elongated, narrow
accumulation of sand oriented parallel to the coast
and separated from the mainland by a lagoon,
estuary, or bay
 Padre Island in Texas is the longest in the U.S.
measuring more than 180 km (112 mi)
– A constantly changing system
 Sea waves dissipate their energy by shifting the
sands and modifying the shape of the island
 Gradually migrate toward the mainland
 Face an open ocean and absorb the brunt of ocean
storms
– Cities such as Atlantic City, NJ, Miami Beach, FL,
and Virginia Beach, VA are built entirely on barrier
islands
– The photographs show a barrier island at Pine
Beach, FL before and after Hurricane Ivan caused
a breach
– Evacuation becomes critical here, as well as other
coastal areas
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Evacuation
 Effectiveness of coastal evacuation
plans was tested in 1985 when
category 3 Hurricane Elena (bottom
figure) followed an erratic path over
the Gulf of Mexico
 The potential downside of evacuation
was illustrated by Hurricane Floyd
(1999), a very large hurricane
approaching the Southeast
– 2 million were evacuated and massive
gridlock occurred
 Greater uncertainty with forecast
track translates into a broader
evacuation zone and greater
economic losses
– Cost of evacuation amounts to about $1
million per mile of coastline
– Vertical evacuation may be an option
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Evacuation
 Other strategies to
minimize loss of life and
property:
– Stringent building codes
– Preservation of mangrove
swamps
– Elimination of federal
floodplain insurance
 The photograph shows a
home designed so that the
first floor will give way to
storm-surge floodwaters.
The second and third floor
living areas are supported
by wooden beams driven
deeply into the sand
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Long-Range Forecasting of Atlantic
Hurricanes
 Since the early 1980s, Prof. William M. Gray and his colleagues at
Colorado State University have issued seasonal hurricane activity
forecasts for the Atlantic basin
– First forecast issued 6 months before the hurricane season and then
updates are made
– Original basis of forecasts was apparent linkage between the frequency of
hurricanes in the tropical Atlantic and rainfall in West Africa
– Also factored in stratospheric quasi-biennial oscillation (QBO)
– After this scheme didn’t work well for several years after giving skillful
results, they developed a new one based on empirical relationships
between Atlantic Basin hurricane activity and a combination of atmospheric
factors in various parts of the world, including the QBO
– In order to improve skill, a modified statistical forecast scheme was
recently developed using data from 1950-2007, and first implemented in
the December 2007 forecast
 NOAA has also issued a Seasonal Outlook for Atlantic basin hurricane
activity since 1998
– Gives probabilities of overall seasonal activity compared to normal, and
likely ranges of named tropical cyclones, hurricanes, major hurricanes, and
© AMS Accumulated Cyclone Energy (ACE)
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Hurricane Modification
 Project STORMFURY (1961-1983)
– Spurred by 6 destructive hurricanes that affected U.S.
East Coast during the mid-1950s
– Working hypothesis: seeding hurricanes with silver
iodide crystals would reduce wind strength
 This was supposed to increase latent heat and enhance
convection just beyond the eye wall
 A new eyewall would then form farther out and the hurricane’s
circulation would theoretically weaken
– Apparent modest successes were dismissed because
convective clouds in hurricanes were found to have too
little supercooled water for seeding to be effective
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